Human milk is recognized as optimal infant nutrition, and increasing the incidence and duration of breast-feeding has been identified as a national health objective. Despite both professional and lay enthusiasm for breast-feeding, many women experience lactation difficulties, usually attributable to breast-feeding mismanagement. However, increasing evidence exists that primary causes of insufficient lactation also occur, although little recognition has been given to primary lactation failure and its incidence remains unknown. The specific aims of the proposed research project are to: 1) determine the percentage of motivated, primiparous women who experience primary lactation failure when breast-feeding healthy infants under optimal circumstances; 2) identify categorical etiologies of insufficient lactation by systematically evaluating lactation performance among women with primary lactation failure and a matched group of controls; and 3) recognize prenatal and early post partum maternal factors which correlate with lactation insufficiency, in order to establish predictors of primary lactation failure. One thousand primiparous women motivated to breast-feed will be recruited in their last month of pregnancy and provided with optimal management in the initiation of lactation. Subjects and their fully breast-fed infants will be followed on the 5th and 10th post partum days to identify those with insufficient lactation manifested by inadequate weight gain in the normal infant. Secondary causes of lactation failure, such as maternal illness and physical complaints, will be distinguished from primary causes of lactation failure. The latter cases and a matched control group of mothers will undergo a detailed investigation of lactation performance, including: 1) measurement of prolactin levels; 2) determination of milk yield by 24 hour infant test-weights; and 3) diaphanography, or light scanning of the breasts. The information acquired by this study should ultimately help to guide infant feeding recommendations, to increase the present understanding of maternal factors involved in the physiologic control of lactation, and to allow early prediction of women at risk for lactation failure.